Sulfur electronegativity

A similar trend is observed as a function of the central chalcogen atoms in the series from sulfur (electronegative and hard) to selenium and tellurium (less electronegative and softer) as central atoms, the symmetry of the XY polyhedra in the solid compounds increases (i.e., the stereochemical activity of the inert pairs decreases) and the tendency to form ionic substructures such as is greatly reduced. 

One reason for the low reactivity of pyridine is that its nitrogen atom because it IS more electronegative than a CH in benzene causes the rr electrons to be held more tightly and raises the activation energy for attack by an electrophile Another is that the nitrogen of pyridine is protonated in sulfuric acid and the resulting pyndinium ion is even more deactivated than pyndine itself... 

H NMR The chemical shift of the proton m the H—C—O—C unit of an ether is very similar to that of the proton m the H—C—OH unit of an alcohol A range of 8 3 2-4 0 IS typical The proton m the H—C—S—C unit of a sulfide appears at higher field than the corresponding proton of an ether because sulfur is less electronegative than oxygen... 

Section 16 18 An H—C—O—C structural unit m an ether resembles an H—C—O—H unit of an alcohol with respect to the C—O stretching frequency m its infrared spectrum and the H—C chemical shift m its H NMR spectrum Because sulfur is less electronegative than oxygen the H and chemical shifts of H—C—S—C units appear at higher field than those of H—C—O—C... 

The biochemical basis for the toxicity of mercury and mercury compounds results from its ability to form covalent bonds readily with sulfur. Prior to reaction with sulfur, however, the mercury must be metabolized to the divalent cation. When the sulfur is in the form of a sulfhydryl (— SH) group, divalent mercury replaces the hydrogen atom to form mercaptides, X—Hg— SR and Hg(SR)2, where X is an electronegative radical and R is protein (36). Sulfhydryl compounds are called mercaptans because of their ability to capture mercury. Even in low concentrations divalent mercury is capable of inactivating sulfhydryl enzymes and thus causes interference with cellular metaboHsm and function (31—34). Mercury also combines with other ligands of physiological importance such as phosphoryl, carboxyl, amide, and amine groups. It is unclear whether these latter interactions contribute to its toxicity (31,36). 

The reaetivity order is pyrrole > fiiran > thiophene, which indicates that electron-donating capacity decreases in the order N > O > The order N > O is as expected on the basis of electronegativity, and O > S probably reflects the better overlap of the oxygen 2p orbital, as compared to the sulfur ip orbital, with the carbon 2p orbitals of the ring. 

Thioesters Like chlorine, sulfur is a third-row element with limited ability to donate a pair of 3p electrons into the carbonyl tt system. With an electronegativity that is much less than Cl or O, however, its destabilizing effect on the carbonyl group is slight, and thioesters lie in the middle of the group of carboxylic acid derivatives in respect to reactivity. 

Both the oxygen and sulfur atoms have two lone pairs while the C/ carbon has ar unpaired electron, and in both cases the double bond shifts from the two carbor atoms to the carbon and the substituent. In acetyl radical, the electron density i centered primarily on the C2 carbon, and the spin density is drawn toward the lattei more than toward the former. In contrast, the density is more balanced between thf two terminal heavy atoms with the sulfur substituent (similar to that in allyl radical with a slight bias toward the sulfur atom. These trends can be easily related to th< varying electronegativity of the heavy atom in the substituent. 

Methanethiol, CH3SH, has a substantial dipole moment [Pg.68]

The hydration reaction just described is typical of what happens when an aldehyde ot ketone is treated with a nucleophile of the type H-Y, where the Y atom is electronegative and can stabilize a negative charge (oxygen, halogen, or sulfur, for instance). In such reactions, the nucleophilic addition is reversible, with the equilibrium generally favoring the carbonyl reactant rather than the tetrahedral addition product. In other words, treatment of an aldehyde or... 

These trends are general ones, observed with other oxoadds of the nonmetals. Recall, for example, that nitric acid, HNO3 (oxid. no. N = +5), is a strong acid, completely ionized in water. In contrast, nitrous add, HN02 (oxid. no. N = +3), is a weak acid (Ka = 6.0 X 10-4). The electronegativity effect shows up with the strengths of the oxoadds of sulfur and selenium ... 

Electronegative groups do not invariably prevent nuclear bromination, but reaction conditions must be much more severe, and the orientation of substitution may be affected by the substituent. Thus 6-nitroquinoline was brominated in sulfuric acid at 100°C to give the 8-bromo product (71) in 51% yield 8-methyl-5-nitroquinoline gave a 69% yield of the 7-bromo derivative (72) under similar conditions, whereas 7-chloroquinoline was transformed into the 5-bromo product (93%) (88CHE892) (Scheme 35). In a sealed tube reaction with bromine, 8-nitroquinoline gave a mixture... 

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